Crude oils contain heteroatomic, polyaromatic molecules that include compounds such as sulfur, nitrogen, nickel, vanadium and others in quantities that can adversely affect the refinery processing of crude oil fractions. Light crude oils or condensates have sulfur concentrations as low as 0.01 percent by weight (W %). In contrast, heavy crude oils and heavy petroleum fractions have sulfur concentrations as high as 5-6 W %. Similarly, the nitrogen content of crude oils can be in the range of 0.001-1.0 W %. These impurities must be removed during refining to meet established environmental regulations for the final products (e.g., gasoline, diesel, fuel oil), or for the intermediate refining streams that are to be processed for further upgrading, such as isomerization or reforming. Furthermore, contaminants such as nitrogen, sulfur and heavy metals are known to deactivate or poison catalysts, and thus must be removed.
Asphaltenes, which are solid in nature and comprise polynuclear aromatics present in the solution of smaller aromatics and resin molecules, are also present in the crude oils and heavy fractions in varying quantities. Asphaltenes do not exist in all of the condensates or in light crude oils; however, they are present in relatively large quantities in heavy crude oils and petroleum fractions. Asphaltene concentrations are defined as the amount of asphaltenes precipitated by addition of an n-paraffin solvent to the feedstock.
In a typical refinery, crude oil is first fractionated in the atmospheric distillation column to separate sour gas including methane, ethane, propanes, butanes and hydrogen sulfide, naphtha (typical boiling point range: 36-180° C.), kerosene (typical boiling point range: 180-240° C.), gas oil (typical boiling point range: 240-370° C.) and atmospheric residue, which are the hydrocarbon fractions boiling above gas oil. The atmospheric residue from the atmospheric distillation column is either used as fuel oil or sent to a vacuum distillation unit, depending upon the configuration of the refinery. Principal products from the vacuum distillation are vacuum gas oil (typical boiling point range: 370-520° C.), and vacuum residue, comprising hydrocarbons boiling above vacuum gas oil.
Vacuum distillation is a well proven technology for physically separating atmospheric residue (AR) into vacuum gas oils (VGO) and vacuum residue (VR). Naphtha, kerosene and gas oil streams derived from crude oils or other natural sources, such as shale oils, bitumens and tar sands, are treated to remove the contaminants, such as sulfur, that exceed the specification set for the end product(s). Hydrotreating is the most common refining technology used to remove these contaminants. Vacuum gas oil is processed in a hydrocracking unit to produce gasoline and diesel, or in a fluid catalytic cracking (FCC) unit to produce mainly gasoline, light cycle oil (LCO) and heavy cycle oil (HCO) as by-products, the former being used as a blending component in either the diesel pool or in fuel oil, the latter being sent directly to the fuel oil pool.
Conventionally, a solvent deasphalting (SDA) process is employed by an oil refinery for the purpose of extracting valuable components from a residual oil feedstock, which is a heavy hydrocarbon that is produced as a by-product of refining crude oil. The extracted components are fed back to the refinery wherein they are converted into valuable lighter fractions such as gasoline, diesel, or lube oil. Suitable residual oil feedstocks which may be used in a SDA process include, for example, atmospheric tower bottoms, vacuum tower bottoms, crude oil, topped crude oils, coal oil extract, shale oils, and oils recovered from tar sands.
Solvent deasphalting (SDA) is used for physical separation of residues by their molecular type. A typical SDA flow scheme is shown in FIG. 1. The key vessel is the extractor where the separation of deasphalted oil (DAO) and pitch occurs. In a typical SDA process, a light hydrocarbon solvent is added to the residual oil feed from a refinery and is processed in what can be termed as an asphaltene separator. Common solvents used comprise light paraffinic solvents. Examples of light paraffinic solvents include, but are not limited to, propane, butane, isobutane, pentane, isopentane, neopentane, hexane, isohexane, heptane, and similar known solvents used in deasphalting, and mixtures thereof. Under elevated temperature and pressures, the mixture in the asphaltene separator separates into a plurality of liquid streams, typically, a substantially asphaltene-free stream of deasphalted oil (DAO), resins and solvent, and a mixture of asphaltene and solvent within which some DAO may be dissolved.
Once the asphaltenes have been removed, the substantially asphaltene-free stream of DAO, resins and solvent is normally subjected to a solvent recovery system. The solvent recovery system of an SDA unit extracts a fraction of the solvent from the solvent rich DAO by utilizing supercritical separation techniques or by boiling off the solvent, commonly using steam or hot oil from fired heaters. The separated solvent is then recycled back for use in the SDA unit.